1. Field of the Invention
This invention relates generally to hydrated lime and method for preparation and use thereof and, more particularly, the invention is directed to an improved process for making hydrated lime having favorable physical properties such as high surface area, high porosity, and small particle size, which in turn provide favorable SO.sub.2 capture properties.
2. Description of Related Technology
Dry sorbent injection technologies offer many advantages over other systems, notably wet flue gas systems, for desulfurization in controlling the emissions of SO.sub.2 produced during combustion of high sulfur coal. Some dry sorbent injection technologies, including furnace sorbent injection (FSI), boiler economizer injection, and post furnace duct-injection/humidification (Coolside) systems have been extensively researched.
A distinguishing factor of dry processes is the injection of a calcium-based sorbent such as hydrated lime (Ca(OH).sub.2) into different locations within a pulverized coal boiler unit. In FSI systems, calcium sulfate is formed by reaction of calcium oxide (CaO) (formed by calcining of calcium carbonate or calcium hydroxide) with SO.sub.2. Under boiler economizer and Coolside conditions, CaSO.sub.3 is the major product of the reaction between Ca(OH).sub.2 and SO.sub.2.
A major objective of research has been to identify sorbent properties that result in enhanced SO.sub.2 capture in order to reduce operating costs and the amount of waste products. In FSI systems, the superiority of calcium hydroxide over calcium carbonate as a sorbent has been attributed to the smaller mean particle size, higher surface area, higher pore volume, larger mean pore diameter, and plate-like grain structure (as opposed to sphere-like grain structure) of CaO derived from Ca(OH).sub.2 compared to that derived from calcium carbonate. In boiler economizer and Coolside studies, improved SO.sub.2 removal performance has been reported for calcium hydroxide with high porosity, high surface area, and small particle size.
Thus, it is desirable to provide calcium hydroxide having high surface area, high porosity, and small particle size for use in SO.sub.2 sorption, and for other physical processes relying on the sorptive capabilities of calcium hydroxide.
In the past, conventional hydrates of calcium produced by slaking lime (CaO) produced by burning limestone (CaCO).sub.3 in a kiln have been limited with respect to these desirable physical properties. Such hydrates have been characterized as having surface areas in the range of about 10-25 m.sup.2 /g, mean particle diameters of about 1.7-10 micrometers, pore volumes in the range of about 0.1-0.25 cc/g, and crystallite sizes of greater than about 200 Angstroms (based on a + c axis dimensions). Several investigators have reportedly prepared small quantities of limes hydrated by contact with an aqueous alcohol solution having surface areas in the range of 35-50 m.sup.2 /g. Bestek, et al. U.S. Pat. No. 4,636,379 (Jan. 13, 1987) describes a seven-stage continuous methanol/water hydration process in which hydrates having surface areas in the range of about 35-55 m.sup.2 /g are reportedly produced. These hydrates are believed to exhibit mean particle diameters of about 1 to about 2, and typically about 1.4, micrometers. Reportedly, very small quantities of hydrates having surface areas as high as 80 m.sup.2 /g have been obtained by calcining (dehydrating) commercial hydrated lime and rehydrating it using an alcohol-water hydration method.
There remains a need in the field for an industrially practical, efficient method of preparing high surface area hydrated lime having all the necessary physical properties for efficient sulfur dioxide sorption, and for other purposes.